Summary of Work: Cellular cations play fundamental roles in hormonal signaling, and are also involved in the mediation of cell injury. The development of intracellular indicators for cytosolic cations and other parameters of interest has had a major impact on the field of cell biology. Our goals in this area have included the development of more sensitive intracellular calcium indicators, and more selective intracellular magnesium indicators. During the last year, several improved fluorinated BAPTA derivatives were developed, which have improved relaxation characteristics, leading to greater sensitivity/unit time, and also additional fluorine nuclei, also resulting in improved sensitivity. Progress on the development of more magnesium selective inhibitors has been slow. A major difficulty in the development of fluorescent chelators is the considerable effort necessary to synthesize them. We have been exploring the use of new methodology to join together, in one step, the fluorophore and the chelator moiety. We have prepared a number of APTRA (o-aminophenol triacetate) derivatives with novel fluorescent groups and are now evaluating their properties. Recent experiments on the transport of fluorinated compounds which are based on the intra/extracellular shift difference phenomenon which we initially observed, have focused on 2'-fluoro-5-methyl-b-L-arabinofuranosyl uracil (L-FMAU), a drug which has been shown to inhibit the growth of Epstein- Barr virus under in vitro conditions. Although the transport of L-FMAU was significantly inhibited (60-70 %) by nitrobenzylthioinosine (NBTI) and dipyridamole, these inhibitors did not achieve complete blockage of L-FMAU uptake. The uptake of L-FMAU was neither reduced in the presence of uracil, a competitor for transport via the nucleobase transporter, nor inhibited by the presence of up to 1000 muM papaverine. Upon inhibition of the nucleoside transporter by NBTI, L-FMAU influx rate increased linearly with its concentration, indicating a non-saturable transport mechanism. The residual transport of L-FMAU (30-40 %) measured in the presence of NBTI was found to be sensitive to butanol which affects membrane fluidity and would be expected to enhance non-facilitated membrane diffusion to the uptake of L-FMAU.